A Group III nitride semiconductor (Group III nitride compound semiconductor or a GaN semiconductor) such as gallium nitride (GaN) has been used widely as materials of various semiconductor elements such as a laser diode (LD) and a light-emitting diode (LED). For example, the laser diode (LD) that emits blue light is applied to a high-density optical disc or a display, and a light-emitting diode (LED) that emits blue light is applied to a display or illumination. Moreover, an ultraviolet LD is expected to be applied to biotechnology and the like, and an ultraviolet LED is expected as an ultraviolet source of a fluorescent lamp.
As a general method for producing a Group III nitride (e.g., GaN) crystal substrate, there is vapor-phase epitaxial growth, for example. As a method capable of producing a Group III nitride single crystal with higher quality, a crystal growth method in a liquid phase is performed. This liquid phase epitaxy method (LPE: Liquid Phase Epitaxy) has had a problem in that the method was required to be performed under high temperature and high pressure. However, with recent improvements, the method could be performed under relatively low temperature and relatively low pressure and became a method suitable for mass production.
A method in which a Group III nitride crystal layer is deposited on a sapphire substrate by an organometallic vapor phase epitaxy (MOCVD: Metalorganic Chemical Vapor Deposition), and thereafter, the Group III nitride crystal thus obtained is further grown by the liquid phase epitaxy method has been reported. Specifically, in Patent Document 1, plural parts of the Group III nitride semiconductor layer formed over the sapphire substrate by MOCVD or the like are selected as seed crystals, and the seed crystals are caused to be in contact with an alkali metal melt to grow the Group III nitride crystal.